Medical devices having alloy compositions

ABSTRACT

A medical device includes an alloy having chromium, niobium, and platinum, wherein the alloy forms at least a portion of the medical device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. application Ser. No. 11/209,940, filed on Aug. 23, 2005, the entirecontents of which are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to medical devices including alloy compositions,and the alloy compositions.

BACKGROUND

The body includes various passageways such as arteries, other bloodvessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced, or even replaced,with a medical endoprosthesis. An endoprosthesis is typically a tubularmember that is placed in a lumen in the body. Examples of endoprosthesesinclude stents, stent-grafts, and covered stents.

An endoprosthesis can be delivered inside the body by a catheter thatsupports the endoprosthesis in a compacted or reduced-size form as theendoprosthesis is transported to a desired site. Upon reaching the site,the endoprosthesis is expanded, for example, so that it can contact thewalls of the lumen.

To support a passageway open, endoprostheses are made of materials, suchas low-carbon, austenitic stainless steel or Nitinol (a nickel-titaniumalloy), having appropriate mechanical properties, such as tensilestrength and yield strength.

When the endoprosthesis is advanced through the body, its progress canbe monitored, e.g., tracked, so that the endoprosthesis can be deliveredproperly to a target site. After the endoprosthesis is delivered to thetarget site, the endoprosthesis can be monitored to determine whether ithas been placed properly and/or is functioning properly.

Methods of tracking and monitoring a medical device include X-rayfluoroscopy and magnetic resonance imaging (MRI). MRI is a non-invasivetechnique that uses a magnetic field and radio waves to image the body.In some MRI procedures, the patient is exposed to a magnetic field,which interacts with certain atoms, e.g., hydrogen atoms, in thepatient's body. Incident radio waves are then directed at the patient.The incident radio waves interact with atoms in the patient's body, andproduce characteristic return radio waves. The return radio waves aredetected by a scanner and processed by a computer to generate an imageof the body.

SUMMARY

The invention relates to medical devices including alloy compositions,and the compositions.

In one aspect of the invention, a medical device includes an alloyincluding chromium, niobium, and platinum, wherein the alloy forms atleast a portion of the medical device.

Embodiments may include one or more of the following features. The alloyincludes less than about 5 percent by weight of a ferromagnetic element,e.g. iron, nickel, or cobalt. The alloy further includes a first elementor a plurality of first elements selected from a group consisting ofsilicon, calcium, boron, aluminum, nitrogen, carbon, selenium, yttrium,tantalum, and manganese. The alloy can include less than about 2% byweight of individual first elements. The alloy includes from about 5percent to about 30 percent by weight platinum, e.g. from about 25percent to about 30 percent by weight platinum. The alloy includes fromabout 5 percent to about 40 percent by weight niobium, e.g. from about10 percent to about 20 percent by weight niobium. The alloy includesfrom about 30 percent to about 90 percent by weight chromium, e.g. fromabout 40 percent to about 50 percent by weight chromium.

The alloy includes from about 30 percent to about 50 percent by weightchromium;

from about 10 percent to about 40 percent by weight niobium; and fromabout 5 percent to about 30 percent by weight platinum. The alloyincludes from about 40 percent to about 50 percent by weight chromium;from about 25 percent to about 30 percent by weight niobium; and fromabout 25 percent to about 30 percent by weight platinum. The alloyconsists essentially of chromium, niobium, and platinum. The alloyincludes a binary phase, e.g. a binary phase is selected from the groupconsisting of Cr₃Pt, Cr₂Nb, and Nb₃Pt.

The device can be in the form of a stent. The device can be selectedfrom the group consisting of a guidewire, a needle, a catheter, anintraluminal filter, a staple, a clip, an orthopedic implant, and dentalprosthesis.

In another aspect of the invention, a stent includes an alloy comprisingfrom about 30 percent to about 50 percent by weight chromium, from about10 percent to about 40 percent by weight niobium, and from about 5percent to about 30 percent by weight platinum, wherein the alloy formsat least a portion of the stent.

Embodiments may include one or more of the following features. The alloyof the stent includes less than about 5 percent by weight of iron,nickel, or cobalt. The alloy of the stent further includes less thanabout 2% by weight of a first element selected from a group consistingof silicon, calcium, boron, aluminum, nitrogen, carbon, selenium,yttrium, tantalum, and manganese. The alloy of the stent includes fromabout 40 percent to about 50 percent by weight chromium, from about 25percent to about 30 percent by weight niobium, and from about 25 percentto about 30 percent by weight platinum. The alloy of the stent includesa binary phase selected from the group consisting of Cr₃Pt, Cr₂Nb, andNb₃Pt.

In another aspect, the invention features an alloy including chromium,niobium, and platinum. In some embodiments, the alloy consistsessentially of chromium, niobium, and platinum, and has less than about5 weight percent (e.g., less than about 4 percent, less than about 3percent, less than about 2 percent, less than about 1 percent, less thanabout 0.5 percent) of any other element.

Embodiments may include one or more of the following features. The alloyincludes less than about 5 percent by weight of a ferromagnetic element,such as iron, nickel, and/or cobalt. The alloy further includes one ormore a first element selected from silicon, calcium, boron, aluminum,nitrogen, carbon, selenium, yttrium, tantalum, and manganese. The alloyincludes a plurality of first elements. The alloy includes less thanabout 2% by weight of the first element. The alloy includes from about 5percent to about 30 percent by weight platinum, for example, from about25 percent to about 30 percent by weight platinum. The alloy includesfrom about 5 percent to about 40 percent by weight niobium, for example,from about 25 percent to about 30 percent by weight niobium. The alloyincludes from about 30 percent to about 85 percent by weight chromium,for example, from about 40 percent to about 50 percent by weightchromium. The alloy includes from about 30 percent to about 50 percentby weight chromium, from about 10 percent to about 40 percent by weightniobium, and from about 5 percent to about 30 percent by weightplatinum. The alloy includes from about 40 percent to about 50 percentby weight chromium, from about 25 percent to about 30 percent by weightniobium, and from about 25 percent to about 30 percent by weightplatinum. The alloy consists essentially of chromium, niobium, andplatinum. The alloy includes one or more binary phases, such as Cr₃Pt,Cr₂Nb, and/or Nb₃Pt.

Embodiments may include one or more of the following advantages. Thealloy compositions have one or more physical and/or mechanicalproperties, such as radiopacity, MRI compatibility (e.g., low magneticsusceptibility), hardness, strength, stiffness (Young's modulus ofelasticity), elongation, and resistance to corrosion, that enhancemedical or non-medical applications. For example, the alloy can beformed into a medical device, such as an endoprosthesis. As a result,the endoprosthesis is capable of having a good balance of yield strengthand stiffness for a tolerable amount of radial recoil upon expansion toallow good apposition of the stent to the vessel wall, good strength tosupport a body, and good radiopacity and MRI compatibility, so that theendoprosthesis can be tracked and monitored. The combination ofproperties allows the alloys to be formed into a variety of products.The alloys can be relatively cost-effective, for example, compared toalloys having high concentrations of precious metal(s).

As used herein, an “alloy” means a substance composed of two or moremetals or of a metal and a nonmetal intimately united, for example, bybeing fused together and dissolving in each other when molten.

Other aspects, features and advantages of the invention will be apparentfrom the description of the preferred embodiments and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an embodiment of a stent.

FIG. 2 is a flow chart of an embodiment of a method of making a stent.

DETAILED DESCRIPTION

Referring to FIG. 1, a stent 20 has the form of a tubular member definedby a plurality of bands 22 and a plurality of connectors 24 that extendbetween and connect adjacent bands. During use, bands 22 are expandedfrom an initial, small diameter to a larger diameter to contact stent 20against a wall of a vessel, thereby maintaining the patency of thevessel. Connectors 24 provide stent 20 with flexibility andconformability that allow the stent to adapt to the contours of thevessel.

Stent 20 includes (e.g., is formed of) an alloy whose compositionincludes chromium, platinum, and niobium. The alloy is capable ofproviding stent 20 with a balance of yield strength and stiffness fortolerable radial recoil upon crimping onto the balloon catheter and uponexpansion in the vessel (for example to have good securement on theballoon catheter while being tracked along the guidewire to theimplantation site and to have good apposition against the vessel wall),strength (for example, to support a body lumen), corrosion resistance,radiopacity, and MRI compatibility. For example, chromium has a highstiffness (Young's modulus of elasticity), is a good solid solutionstrengthener, and aids in corrosion resistance. Platinum also is a goodsolid solution strengthener, aids in corrosion resistance, as well asprovides a high mass absorption coefficient for enhanced radiopacity.Niobium has a low magnetic susceptibility and is compatible with (e.g.,soluble in) chromium and platinum. Because niobium also has goodradiopacity, inclusion of niobium allows a reduction in the amount ofplatinum used to achieve a given radiopacity. Furthermore, in someembodiments, the alloy includes less than about 5 percent by weight(e.g., less than about 4 percent, less than about 3 percent, less thanabout 2 percent, less than about 1 percent, less than about 0.5 percent)of ferromagnetic materials, such as iron, nickel, and/or cobalt. Withoutwishing to be bound by theory, it is believed that the limited amountsof ferromagnetic materials in the alloy reduce (e.g., minimize oreliminate) interference with MRI techniques, thereby allowing goodvisualization of stent 20, material (such as blood and tissue) withinthe lumen of the stent, and material surrounding the stent.

Without wishing to be bound by theory, it is believed that chromium canenhance the corrosion resistance of the alloys, e.g., by increasing thepitting resistance of the alloy. For example, in certain alloys,chromium can form a thin oxide layer on the surface of an alloy thatenhances the resistance of the alloy to corrosive attack. The degree ofcorrosion resistance can be a function of the chromium concentration andthe concentrations of other elements in the alloy. The alloy can includefrom about 30 to about 85 weight percent of chromium. The alloy caninclude greater than or equal to about 30, about 35, about 40, about 45,about 50, about 55, about 60, about 65, about 70, or about 75 weightpercent, and/or less than or equal to about 85, about 80, about 75,about 70, about 65, about 60, about 55, about 50, about 45, about 40, orabout 35 weight percent of chromium.

Niobium can enhance the radiopacity of the alloy and provide the alloywith a low magnetic susceptibility. In some embodiments, the alloyincludes from about 5 to about 40 weight percent of niobium. Forexample, the alloys can include greater than or equal to about 10, about12.5, about 15, about 17.5, about 20, about 22.5, about 25, about 27.5,about 30, about 32.5, about 35 or about 37.5 weight percent, and/or lessthan or equal to about 40, about 37.5, about 35, about 32.5, about 30,about 27.5, about 25, about 22.5, about 20, about 17.5, about 15, orabout 12.5 weight percent of niobium.

Platinum can also enhance the radiopacity of the alloy, as well asprovide strength and corrosion resistance. In embodiments, the alloyincludes from about 5 to about 30 weight percent of platinum. Forexample, the alloy can include greater than or equal to about 5, about7.5, about 10, about 12.5, about 15, about 17.5, about 20, about 22.5,about 25, or about 27.5 weight percent, and/or less than or equal toabout 30, about 27.5, about 25, about 22.5, about 20, about 17.5, about15, about 12.5, about 10, or about 7.5 weight percent of platinum.

In addition to chromium, niobium, and platinum, the alloy can furtherinclude one or more (e.g., two, three, four, five, six or more)additional elements capable of assisting with phase stabilization,microcleanliness, and hot workability. Examples of additional element(s)include silicon, calcium, boron, aluminum, nitrogen, carbon, selenium,yttrium, tantalum, and manganese. Each individual additional element canbe present up to about 2 percent (e.g., greater than or equal to 0.25percent, 0.50 percent, 1.0 percent, 1.25 percent, 1.50 percent, or 1.75percent) by weight in the alloy. In some embodiments, the alloy includesa total of from about 0.10 to about 5.00 weight percent of one or moreadditional elements.

The alloy can further include one or more microalloyed elements orresidual amounts of impurities elements. For example, the alloy mayinclude phosphorus (e.g., 0.025 wt % maximum), sulfur (e.g., 0.010 wt %maximum), vanadium (e.g., about 0.07 wt %), titanium (e.g., 0.002 wt %),and/or copper (e.g., about 0.2 wt %). Other microalloyed and residualelements are possible, which can be a function of the source of thematerials.

The alloy can include substantially one homogeneous phase, or includetwo or more discrete phases. Examples of additional phases includebinary phases, such as Cr₃Pt, Cr₂Nb, and/or Nb₃Pt, that can enhance theyield strength of the alloy. The binary phase(s) can be precipitated inthe alloy by heat treatment. In some embodiments, the alloy includesfrom about 1 to about 25 percent of one or more binary phases in theplanar area observed in a 1,000× field of view.

The alloy can have a microstructure that is predominantly (greater than50%) a single-phase solid solution of chromium-platinum-niobium. It isbelieved that the single-phase microstructure provides the alloy withhigher strength and ductility relative to pure chromium.

The alloy can have high corrosion resistance. The relative pittingcorrosion resistance can be compared using a pitting resistanceequivalent (PRE) as is done with stainless steels, which can becalculated as

PRE=% Cr+3.3×% Mo

ASTM F138 316L grade stainless steel for surgical implants is requiredto have a PRE≧26.0. This alloy is known to have excellentbiocompatibility and corrosion resistance. Cr-10Nb-10Pt does not containMo for enhancing pitting corrosion resistance, but contains platinumwhich is also known to enhance corrosion resistance in chromium (e.g.,Alloys cathodically modified with noble metals, Reviews of AppliedElectrochemistry 28, J. H. Potgeiter, Journal of AppliedElectrochemistry 21 (1991) 471-482). More information about PREs can befound in S.D. Kiser, Preventing Weld Corrosion, Advanced Materials &Processes, March 2002, pp. 32-35.

The alloy can also have high hardness and/or high strength. In someembodiments, the alloy has a hardness greater than about 60 Rockwell Be.g., greater than about 65, 70, or 75 Rockwell B. The alloy can have aYoung's modulus of elasticity (E) of greater than about 25 msi, e.g.,greater than about 28, 30, or 32 msi. The alloy can have an ultimatetensile strength (UTS) of greater than about 60 ksi, e.g., greater thanabout 70, 80, or 90 ksi. The alloy can have a 0.2% offset yield strength(YS) of greater than about 30 ksi, e.g., 40, 50, or 60 ksi. The alloycan have a percent elongation (% el) of greater than about 10% el, e.g.,15, 20, or 25% el.

Referring to FIG. 2, a method 40 of making stent 20 is shown. Method 40includes forming a tube (step 42) including the alloy that makes up thetubular member of stent 20. The tube is subsequently cut to form bands22 and connectors 24 (step 44) to produce an unfinished stent. Areas ofthe unfinished stent affected by the cutting may be subsequently removed(step 46). The unfinished stent may be finished to form stent 20 (step48).

The alloy can be synthesized by intimately combining the components ofthe alloy. For example, a targeted alloy composition can be formed bymelting elemental bits or powders in the appropriate concentrations.Melting can be performed at temperatures above about 1800° C. for 10 to180 minutes using vacuum induction melting (VIM), vacuum arc remelting(VAR), electron beam melting (EBM), plasma melting, vacuum or inert gasplasma deposition. Alloying can be performed in the solid state byblending elemental powders and hot isostatic pressing at temperaturesgreater than about 1000° C. and less than about 1500° C. for 12 to 36hours at 5 to 40 ksi pressure, and/or cold pressing and sintering attemperatures greater than about 1000° C. and less than about 1500° C.for 4 to 60 hours. The alloy can be in the form of an ingot, a compact,or a deposit that is subsequently shaped into a feedstock, such as ahollow tubular member. In some embodiments, the alloy is processed(e.g., by heat treatment at 1200° C. for six hours) to homogenize thealloy and/or to yield an alloy with a selected structure and properties.

In some embodiments, the hollow tubular member including the alloy canbe drawn through a series of dies with progressively smaller circularopenings to plastically deform the member to a targeted size and shape.The plastic deformation strain can harden the member (and increases itsyield strength) and elongate the grains along the longitudinal axis ofthe member. The deformed member can be heat treated (e.g., annealedbelow or above the recrystallization temperature) to transform theelongated grain structure into a partial or fully recrystallized grainstructure, e.g., one including equiaxed grains. Small or fine grains canbe formed by heating the member close to the recrystallizationtemperature for a short time. Large or coarse grains can be formed byheating the member at higher temperatures and/or for longer times topromote grain growth.

Next, bands 22 and connectors 24 of stent 20 are formed, as shown, bycutting the tube (step 44). Selected portions of the tube can be removedto form bands 22 and connectors 24 by laser cutting, as described inU.S. Pat. No. 5,780,807, hereby incorporated by reference in itsentirety. In certain embodiments, during laser cutting, a liquidcarrier, such as a solvent or an oil, is flowed through the lumen of thetube. The carrier can prevent dross formed on one portion of the tubefrom re-depositing on another portion, and/or reduce formation of recastmaterial on the tube. Other methods of removing portions of the tube canbe used, such as mechanical machining (e.g., micro-machining),electrical discharge machining (EDM), and photoetching (e.g., acidphotoetching).

In some embodiments, after bands 22 and connectors 24 are formed, areasof the tube affected by the cutting operation above can be removed (step46). For example, laser machining of bands 22 and connectors 24 canleave a surface layer of melted and resolidified material and/oroxidized metal that can adversely affect the mechanical properties andperformance of stent 20. The affected areas can be removed mechanically(such as by grit blasting or honing) and/or chemically (such as byetching or electropolishing). In some embodiments, the tubular membercan be near net shape configuration after step 46 is performed.“Near-net size” means that the tube has a relatively thin envelope ofmaterial that is removed to provide a finished stent. In someembodiments, the tube is formed less than about 25% oversized, e.g.,less than about 15%, 10%, or 5% oversized.

The unfinished stent is then finished to form stent 20 (step 48). Theunfinished stent can be finished, for example, by electropolishing to asmooth finish. Since the unfinished stent can be formed to near-netsize, relatively little of the unfinished stent need to be removed tofinish the stent. As a result, further processing (which can damage thestent) and costly materials can be reduced. In some embodiments, about0.0001 inch of the stent material can be removed by chemical millingand/or electropolishing to yield a stent.

Stent 20 can be of a desired shape and size (e.g., coronary stents,aortic stents, peripheral vascular stents, gastrointestinal stents,urology stents, and neurology stents). Depending on the application,stent 20 can have a diameter of between, for example, 1 mm to 46 mm. Incertain embodiments, a coronary stent can have an expanded diameter offrom about 2 mm to about 6 mm. In some embodiments, a peripheral stentcan have an expanded diameter of from about 5 mm to about 24 mm. Incertain embodiments, a gastrointestinal and/or urology stent can have anexpanded diameter of from about 6 mm to about 30 mm. In someembodiments, a neurology stent can have an expanded diameter of fromabout 1 mm to about 12 mm. An abdominal aortic aneurysm (AAA) stent anda thoracic aortic aneurysm (TAA) stent can have a diameter from about 20mm to about 46 mm. Stent 20 can be balloon-expandable, or a combinationof self-expandable and balloon-expandable (e.g., as described in U.S.Pat. No. 5,366,504).

In use, stent 20 can be used, e.g., delivered and expanded, using acatheter delivery system. Catheter systems are described in, forexample, Wang U.S. Pat. No. 5,195,969, Hamlin U.S. Pat. No. 5,270,086,and Raeder-Devens, U.S. Pat. No. 6,726,712. Stents and stent deliveryare also exemplified by the Radius® or Symbiot® systems, available fromBoston Scientific Scimed, Maple Grove, Minn.

While a number of embodiments have been described above, the inventionis not so limited.

As an example, while stent 20 is shown as being formed wholly of thealloy, in other embodiments, the alloy forms one or more selectedportions of the medical device. For example, stent 20 can includemultiple layers in which one or more layers include the alloy, and oneor more layers do not include the alloy, e.g., 316L stainless steel.Stents including multiple layers are described, for example, inpublished patent application 2004-0044397, and Heath, U.S. Pat. No.6,287,331.

Stent 20 can be a part of a covered stent or a stent-graft. For example,stent 20 can include and/or be attached to a biocompatible, non-porousor semi-porous polymer matrix made of polytetrafluoroethylene (PTFE),expanded PTFE, polyethylene, urethane, or polypropylene.

Stent 20 can include a releasable therapeutic agent, drug, or apharmaceutically active compound, such as described in U.S. Pat. No.5,674,242, U.S. Ser. No. 09/895,415, filed Jul. 2, 2001, and U.S. Ser.No. 10/232,265, filed Aug. 30, 2002. The therapeutic agents, drugs, orpharmaceutically active compounds can include, for example,anti-thrombogenic agents, antioxidants, anti-inflammatory agents,anesthetic agents, anti-coagulants, and antibiotics.

In some embodiments, a stent can be formed by fabricating a wireincluding the alloy, and knitting and/or weaving the wire into a tubularmember.

The alloys can be used to form other medical devices, such as those thatbenefit from having high strength to resist overloading and fracture,high corrosion resistance, and/or biocompatibility (e.g., capable ofbeing implanted in a body for long periods (such as greater than tenyears)), particularly medical implants and devices that will be usedwith fluoroscopy and/or MRI during a medical procedure or when patientswill be subjected to follow-up MRI imagery. For example, the alloys canbe used to manufacture other endoprostheses. The alloys can be used infilters such as removable thrombus filters described in Kim et al., U.S.Pat. No. 6,146,404, which is hereby incorporated by reference; inintravascular filters such as those described in Daniel et al., U.S.Pat. No. 6,171,327, which is hereby incorporated by reference; and venacava filters such as those described in Soon et al., U.S. Pat. No.6,342,062, which is hereby incorporated by reference.

The alloys can be used to form a guidewire (such as a Meier SteerableGuide Wire (for AAA stent procedure)), an ASAP Automated Biopsy System(e.g., for a stylet and/or a cannula, as described in U.S. Pat. Nos.4,958,625, 5,368,045, and 5,090,419), or a hypotube of a catheter (e.g.,a balloon catheter).

The alloys can also be used to manufacture cutting elements, such asthose carried by a medical balloon catheter described in U.S. Ser. No.10/335,604, filed Jan. 2, 2003, and U.S. Pat. No. 5,209,799, and U.S.Pat. No. 5,336,234. The hardness and strength of the alloys can reduceedge rounding (which can decrease sharpness) and deformation of theproduct shape. Also, in some cases, the relatively high corrosionresistance of the alloys allows the instruments to be exposed torepeated steam autoclave sterilization cycles. As a result, theinstruments can be reused more, and the cost of replacement is reduced.

Still other examples of medical devices include, needles, catheters,staples, wires used for wound closure, clips, orthopedic devices (suchas hip stems and knee trays), and dental prostheses.

The following examples are illustrative and not intended to be limiting.

EXAMPLE 1 Cr-10Nb-10Pt Strip Material and Stent Manufacture

An arc melter was used to make a Cr-10Nb-10Pt alloy ingot.

The Cr-10Nb-10Pt ingot was made by melting high purity virgin elementalcharge materials. The charge materials were procured from GoodfellowCambridge Limited.

TABLE 1 Goodfellow Material P/N Purity Form Dimension Temper ChromiumCR006115 99.95% Pellets 10 mm N/A Niobium NB007910 99.9% Rod  2 mm dia.annealed Platinum PT005156 99.99% Wire  1 mm dia. annealed

Table 1 shows the charge materials used in Cr-10Nb-10Pt alloy melting.The pure charge materials were cut and weighed according to an aim alloyformulation of 80 weight percent chromium, 10 weight percent niobium,and 10 weight percent platinum for a total ingot weight of 70 grams.After cleaning with alcohol, the charge materials were loaded into thewater-cooled copper hearth plate ingot cavity of the arc meltingfurnace: a Materials Research Furnaces (MRF) model ABJ-900 inert-gas arcmelter. The arc melter consisted of a water cooled copper hearth platewith multiple cavities for charge material placement. Once loaded, thecharge materials and copper hearth were sealed inside a bell chamber formelting. After the chamber was evacuated with a vacuum pump and argonback-filled multiple times, the arc melter's power supply was startedand a tungsten electrode was used to strike an arc with the chargematerials and copper hearth. Melting was performed while using a maximumpower supply current output of 400 A. During melting, a constant flow(10 to 30 L/min) of high purity argon gas was passed through the chamberto protect the charge metal from atmospheric contaminants (e.g., O andN). The charge materials were heated and melted until they appeared tohave completely mixed to form one solid ingot. Once melting wascomplete, the newly formed ingot was allowed to cool under protection ofthe argon cover gas before removing it from the furnace chamber. Theingot was remelted twice in an attempt to homogeneously mix the chargematerials.

After melting, the Cr-10Nb-10Pt ingot measured approximately 3.5″long×0.6″ wide×0.3″ thick. The ingot was vacuum annealed at 1200° C. for1 hour. The heat treatment was performed to bring into solution anysecond phases that may have formed during ingot solidification andcooling in the arc melter. The ingot was then machined to a thickness of0.2″ for hot rolling.

Binary phase diagrams were reviewed for Cr—Nb, Cr—Pt, and Nb—Pt. Table 2shows the liquidus temperatures obtained from the phase diagrams.

TABLE 2 Element or Alloy: Liquidus Temperature, ° C. Cr 1875 Nb 2468 Pt1769 Cr—10Pt 1700 Cr—10Nb 1800 50Pt—50Nb 1950

The dendritic ingot microstructure was expected to consist of Cr—Nb—Ptdendrite arms and Nb—Pt enriched interdendritic regions. This hypothesisis based upon the liquidus temperatures which indicate that if elementalsegregation occurs, chromium-niobium-platinum alloy should solidifyfirst and Nb—Pt alloy would solidify last upon cooling from the liquidphase.

According to the Nb—Pt phase diagram, the solid phase interdendriticregion could be Nb₃Pt or Nb₂Pt intermetallics. This material wouldlikely be brittle. If the ingot were strained, fracture would likelyoccur through the intermetallics with little plastic deformation.

Microstructural analysis of the as-cast ingot revealed a dendriticmicrostructure with the dendritic arms being darker than theinterdendritic regions when imaged with backscattered electrons in theScanning Electron Microscope (SEM). Brighter areas are typicallyassociated with heavy elements when imaged in this manner. SEM EnergyDispersive X-ray Spectroscopy (EDS) spectra taken of the dendrite armsand interdendritic regions revealed that the dendrites werechromium-rich Cr—Nb—Pt and the interdendritic regions were Nb—Pt richCr—Nb—Pt.

The ASTM E384 Vickers microhardness of the ingot microstructuralspecimen was measured and converted to 77 Rockwell B. This converts to aroughly estimated ultimate tensile strength of about 68 ksi usingconversion tables for steels since one is not available for chromiumalloys.

In order to be able to hot or cold work the ingot to produce strip formaterials characterization testing or stent tubing fabrication, theingot was homogenization heat treated to reduce the elementalconcentration gradients between the dendrites and interdendritic regionsand thereby eliminate the brittle intermetallic phase(s). Homogenizationof the ingot was performed in a partial pressure of argon gas at 1200°C. for 18 hours. The partial pressure of argon gas is intended tominimize the chromium vaporization that would occur at 1200° C. in highvacuum. The process is to perform metallography on the homogenizedmaterial after 6, 12, and 18 hours to see when the interdendriticregions appears less segregated via SEM backscattered electroncompositional imaging.

When the concentration gradient qualitatively looks significantlyreduced, hot rolling is initiated. Hot rolling of the ingot can beperformed by heating to red hot in air or with the ingot encapsulated by0.006″ thick 316 stainless steel strip and passed through rollers set aincrementally smaller gaps until a thickness of 0.025″ is obtained.Between 15% total reductions, the ingot is annealed or stress reliefheat treated to eliminate or reduce strain hardening produced duringrolling and to return the material to a plastically deformablecondition.

After rolling, the 0.0052″ thick strip can be machined to produce flat“dog-bone” shaped specimens for tensile testing. Stent strut patternscan be laser machined into the strip. Post-laser dross removal andelectropolishing can be performed to bring the strut pattern to finisheddimensions. The strip can then be rolled into the tubular stent shapeand crimped onto a balloon catheter.

EXAMPLE 2 Cr-10Nb-10Pt Seamless Tubing and Stent Manufacture

A total of 20 pounds of niobium, platinum, and chromium can be weighedout for the alloy formulation of 80 weight percent chromium, 10 weightpercent niobium, and 10 weight percent platinum. The charge materialscan be loaded into the hearth of a vacuum electron beam melter andmelted to form the liquid alloy. Upon solidification and cooling below200° C., the ingot would be removed from the EB melter and subjected totwo vacuum arc remelting operations to refine the cast microstructureand improve microcleanliness. The triple melted ingot can be coated withglass lubricant and charged into an extrusion press and heated to 1250°C. The hydraulic extrusion press can be used to convert the 3 inchdiameter ingot to a 2.5 inch diameter billet. A second extrusionoperation can be performed convert the 2.5 inch diameter billet to a 2inch diameter billet. The billet can then be homogenized at 1200° C. for18 hours in a partial pressure of argon gas. The homogenized billet canthen be gun drilled to produce a 1.0″ diameter inner diameter along thelongitudinal centerline. The billet outer diameter can be machined to1.8″ diameter such that it is highly concentric with the gun drilledinner diameter.

The hollow rod can be pilgered at room temperature to reduce the OD to1.25″ diameter with 1200° C./1-hour 10% reduction in diameter interpassanneals in a partial pressure of argon gas. The 1.25″ diameter pilgeredtubing can then be mandrel drawn in straight lengths with 1200°C./30-minute 10% reduction in diameter interpass anneals in a partialpressure of argon gas. At 0.6643″ OD, the mandrel drawn tubing can befloating plug drawn in straight lengths with 1200° C./30-minute 10%reduction in diameter interpass anneals in a partial pressure of argongas until an OD of 0.083″ and a wall thickness of 0.0052″ are reached.

Stent strut patterns can be laser machined into the seamless drawntubing. Post-laser dross removal and electropolishing can be performedto produce finished stent dimensions. The stents can be crimped ontoballoon catheters.

All publications, references, applications, and patents referred toherein are incorporated by reference in their entirety.

Other embodiments are within the claims.

1-24. (canceled)
 25. A medical device comprising an alloy comprisingchromium, greater than or equal to about 5 weight percent niobium, andgreater than or equal to about 5 weight percent platinum, wherein thealloy forms at least a portion of the medical device.
 26. The device ofclaim 25, wherein the alloy comprises less than about 5 percent byweight of a ferromagnetic element.
 27. The device of claim 25, whereinthe alloy comprises less than about 5 percent by weight of iron, nickel,or cobalt.
 28. The device of claim 25, wherein the alloy furthercomprises a first element selected from a group consisting of silicon,calcium, boron, aluminum, nitrogen, carbon, selenium, yttrium, tantalum,and manganese.
 29. The device of claim 28, wherein the alloy comprises aplurality of first elements.
 30. The device of claim 28, wherein thealloy comprises less than about 2% by weight of the first element. 31.The device of claim 25, wherein the alloy comprises from about 5 percentto about 30 percent by weight platinum.
 32. The device of claim 25,wherein the alloy comprises from about 25 percent to about 30 percent byweight platinum.
 33. The device of claim 25, wherein the alloy comprisesfrom about 5 percent to about 40 percent by weight niobium.
 34. Thedevice of claim 25, wherein the alloy comprises from about 10 percent toabout 20 percent by weight niobium.
 35. The device of claim 25, whereinthe alloy comprises from about 30 percent to about 90 percent by weightchromium.
 36. The device of claim 25, wherein the alloy comprises fromabout 40 percent to about 50 percent by weight chromium.
 37. The deviceof claim 25, wherein the alloy comprises: from about 30 percent to about50 percent by weight chromium; from about 10 percent to about 40 percentby weight niobium; and from about 5 percent to about 30 percent byweight platinum.
 38. The device of claim 25, wherein the alloycomprises: from about 40 percent to about 50 percent by weight chromium;from about 25 percent to about 30 percent by weight niobium; and fromabout 25 percent to about 30 percent by weight platinum.
 39. The deviceof claim 25, wherein the alloy consists essentially of chromium,niobium, and platinum.
 40. The device of claim 25, wherein the alloycomprises a binary phase.
 41. The device of claim 40, wherein the binaryphase is selected from the group consisting of Cr₃Pt, Cr₂Nb, and Nb₃Pt.42. The device of claim 25, in the form of a stent.
 43. The device ofclaim 25, wherein the device is selected from the group consisting of aguidewire, a needle, a catheter, an intraluminal filter, a staple, aclip, an orthopedic implant, and dental prosthesis.
 44. A stent,comprising an alloy comprising chromium, greater than or equal to about10 weight percent niobium, and greater than or equal to about 5 weightpercent platinum, wherein the alloy forms at least a portion of thestent.